1. Field of the Invention
It is common practice to utilize a steel canister as the inflator pressure vessel of an automobile occupant restraint system because of the relatively high strength of steel at elevated temperatures. However, emphasis on vehicle weight reduction has renewed interest in the use of aluminum in place of steel in such pressure vessels.
One of the tests vehicle occupant restraint inflator systems must pass is exposure to fire whereupon the gas generating material of the inflator is expected to ignite and burn but the inflator pressure vessel must not rupture or throw fragments. With steel pressure vessels, this test was relatively easy to pass because steel retains most of its strength at ambient temperatures well above the temperature at which the gas generant autoignites. Aluminum, however, loses strength rapidly with increasing temperature and may not be able to withstand the combination of high ambient temperature and high internal temperature and pressure generated upon ignition of the gas generant. If, however, the gas generant of the inflator can be made to autoignite at relatively low temperatures, for example, 150.degree. C. to 210.degree. C., the inflator canisters can be made of aluminum.
2. Description of the Prior Art
One patent related to the subject matter of this invention is U.S. Pat. No. 4,561,675 granted to Adams et al. This patent discloses the use of Dupont 3031 single base smokeless powder as an autoignition gas generant. However, smokeless powder autoignites by a different mechanism than the compositions of the instant invention. Moreover, while such smokeless powder autoignites at approximately the desired temperature of 177.degree. C., it is largely composed of nitrocellulose. It is well known in the propellant field that nitrocellulose is not stable for long periods at high ambient temperatures.